nagios: use hash instead of array for hosts

--- git/bitmask ‹develop* M› » leap deploy wallaby                                                                                      1 ↵
 = checking node
   - [wallaby] ok
 = synching configuration files
   - hiera/wallaby.yaml -> wallaby:/etc/leap/hiera.yaml
   - files/branding/favicon.ico, files/branding/tail.scss, files/branding/head.scss, files/branding/img/ -> wallaby:/etc/leap
 = synching puppet manifests
   - /home/varac/dev/projects/leap/git/leap_platform/[bin,puppet] -> wallaby:/srv/leap
 = applying puppet
   - [wallaby] notice: Scope(Class[main]): Services for wallaby.bitmask.net: monitor webapp
   - [wallaby] Resource title must be a String, not Hash at /srv/leap/puppet/modules/site_nagios/manifests/server.pp:38 on node wallaby.bitmask.net
   - [wallaby] Puppet apply complete (failed).

(from redmine: created on 2013-09-23, closed on 2013-10-18, blocks #4169 (closed))